A conventional screw driver generally includes a handle and a shank which includes a polygonal recess defined in a distal end thereof so as to receive screw bits. The user rotates the handle to output a torque from the screw bit so as to tighten or loosen a bolt or a screw. The output torque of the conventional screw driver cannot be controlled so that the conventional screw driver cannot be used on some machines that require precise torque for the screws. Although some screw drivers have the feature of adjustable torque, most of them have a common shortcoming which is that the screw driver provides a fixed resistance and this is obviously not satisfied for the users.
The present invention intends to provide a screw driver that includes several torques to be chosen and the user may fix the screw driver at a specific position which provides a pre-set amount of torque.